In general, in a water cooled engine of a motor vehicle, the engine is connected to a radiator by a cooling water circuit. Cooling water that is cooled down by a radiator is supplied to the water cooled engine through a cooling water supply passage, and the cooling water that is heated by the engine is returned through the cooling water return passage to the radiator, where the cooling water is cooled down due to heat transfer between the cooling water and the open air.
In such a water cooled engine, there is a problem in that startability of the engine deteriorates because the engine is cooled down to the same temperature as the outside air temperature while a motor vehicle is parked during the winter season and in a cold region. In addition, there is another problem in that fuel efficiency thereof decreases because it takes time to rise the temperature of the engine up to an appropriate one at cold start of the engine. For instance, in an engine equipped with an electronic fuel injection system, the engine is maintained to run in a state where idle speed thereof is high, so that the fuel efficiency deteriorates. In order to shorten the time required for the engine to reach a desired operating temperature, the radiator can be bypassed so that the cooling water is prevented from being cooled during a cold start.
At the same time, the function of other components connected to the coolant circuit, such as heat exchangers for an exhaust gas recirculation (EGR) system, a transmission connected to the engine or a catalytic converter, such as a selective catalytic reduction (SCR) device, for exhaust after-treatment can have different cooling requirements during a cold start and normal operation. For instance, relatively cold coolant flowing through an SCR heat exchanger or a transmission cooler during a cold start will delay the point in time when such components reach their operating temperature. However, after a cold start the same components can require effective cooling.